Shadow Mishima looks exactly like his real self but his eyes are golden.
Shadow Mishima is born from the real Mishima's repressed feelings of wanting to get revenge against his bullies using his Phantom Aficionado Website to threaten them both online and offline and his desire to use the website for his own popularity.
Shadow Mishima is Yuuki Mishima's Shadow Self and appears in Mementos. The Phantom Thieves of Hearts notice this and confront him. The protagonist helps Shadow Mishima face himself and once Shadow Mishima realizes he's using the Phantom Thieves the real Mishima also realizes this and regrets it. The real Mishima decides to use the Phan-site to let the people's voice be heard to the Phantom Thieves.